Almost had to share air three days ago.
My buddy was a DM, and a good one. I'm an instructor, but we were on his turf and diving from the boat he captains, so he led the dive.
Lionfish hunting in 60 feet of water, I had crawled under a ledge and was wedged in about as far I could get with just my fins sticking out. My buddy was shining his light on my target through a crack that was too small to get more than a hand though.
I was just starting to wonder if I might have gone in farther than was prudent when, suddenly, I heard a WHOOOSH and the steady beam of his light started waving back and worth.
I backed out as quickly as I could. When I emerged, his head was in a cloud of bubbles flowing from his first stage, and he was giving the OOA signal.
I offered my air, but he showed me his SPG instead of accepting the reg. It was at a little more than 20 bar and going down visibly but slowly. The volume of bubbles around his head seemed to decreasing as his tank pressure decreased.
He had the presence of mind to finish killing the last lionfish I had shot, and we ascended in the share-air position. We used my octo to inflate his smb. We cut the safety stop off after one minute, which we were comfortable doing because we had been down only for 20 minutes with a max depth of 63 feet. He ran out of air just as we surfaced.
He said his SPG went from 120 to 40 bar in the first few seconds of the emergency, and his biggest concern was about leaving me stuck under the ledge if he had to do a CESA.
For my part, I was confident I could eventually get out because I had kept one arm in front of my head and the other back down my waist so I'd always be able to push or pull with at least one hand. My chief concern was that his signal meant he was already out of air. He explained that he gave me OOA instead of LOA because he was losing pressure at a rate that meant he would be out of of air in just a few seconds and he wanted an OOA response from me. Once he showed me his SPG, I understood better what was happening.
Later that night over lionfish ceviche, some beers, some rum and some more rum, we regaled my wife and his fiancee with tales of each other's calm reactions to the emergency, thus completing the debriefing and bonding experience. We decided we'd be happy with each other as buddies the next time something goes wrong.
The lionfish died, and we lived. It was a good dive.
Haven't yet heard what failed in his regulator.